Speech coders are known in the art. Some speech coders convert analog voice samples into digitized representations, and subsequently represent the spectral speech information through use of linear predictive coding. Other speech coders improve upon ordinary linear predictive coding techniques by providing an excitation signal that is related to the original voice signal.
U.S. Pat. No. 4,817,157 describes a digital speech coder having an improved vector excitation source wherein a codebook of codebook excitation vectors is accessed to select an codebook excitation signal that best fits the available information, and is used to provide a recovered speech signal that closely represents the original. In such a system, pitch excitation information and codebook excitation information are developed and combined to provide a composite signal that is then used to develop the recovered speech information. Prior to combination of these signals, a gain factor is applied to each, to cause the amount of energy associated with each signal to be representational of the amount of energy associated with the original voice components represented by these constituent parts.
The speech coder determines the appropriate gain factors at the time of determining the appropriate pitch excitation and codebook excitation information, and coded information regarding all of these elements is then provided to the decoder to allow reconstruction of the original speech information. In general, prior art speech coders have provided this gain factor information to the decoder in discrete form. This has been accomplished either by transmitting the information in separate identifiable packets, or in other form (such as by vector quantization) where, though combined for purposes of transmission, are still effectively independent from one another.
Prior art speech coding techniques leave considerable room for improvement. The gain factor transmission methodology referred to above may require a considerable amount of transmission medium capacity to accomodate error protection (otherwise, errors that occur during transmission will corrupt the gain information, and this can result in extremely annoying incorrect speech reproduction results).
Accordingly, a need exists for a method of speech coding that reduces demands on the transmission medium, while simultaneously providing increased protection for gain factor information.